On Thursday, the White House acknowledged that an American hostage, Warren Weinstein, had been inadvertently killed in a U.S. operation earlier this year. Weinstein, along with an Italian hostage named Giovanni Lo Porto, died in what has been described as a drone strike against al-Qaeda militants along Pakistan's border with Afghanistan.

American hostages have died as a result of U.S. military action before – just last year, journalist Luke Somers was fatally shot while U.S. Navy SEALs tried to rescue him from al-Qaeda captivity in Yemen. But Weinstein appears to be the first hostage killed by the U.S. government's controversial drone operations.

Worse still, his death wasn't revealed by the U.S. government for months, partly because it took a long investigation to actually confirm it, according to the Wall Street Journal. It's enough to make you wonder how many Americans have been killed by U.S. drone strikes – and how many were killed on purpose.

Americans have been killed by U.S. drone strikes from the very beginning. In 2002, in an operation coordinated by the U.S. military, U.S. citizen Kemal Darwish was reported to have been killed in a strike in Yemen. In 2013, the U.S. Justice Department confirmed that four U.S. citizens, including cleric Anwar al-Awlaki, had been killed by CIA drone strikes since 2009. Awlaki, a U.S.-born Muslim known as a gifted preacher who incited attacks against the West, was killed in 2011 in a strike in Yemen.

Another U.S. citizen, Samir Khan, died in the same strike that killed Awlaki, the Justice Department revealed. Khan was an al-Qaeda militant who created and served as an editor for Inspire, the group's magazine. Awlaki's son, Abdulrahman al-Awlaki, was killed in another attack the month after his father died. The 16-year-old was also a U.S. citizen.

The other death announced by the Justice Department in 2013 was that of Jude Kenan Mohammad, a 23-year-old who was born in Florida. He was killed in a U.S. drone strike in Pakistan in 2011. Mohammad had been acting as a recruiter for al-Qaeda and the Pakistani Taliban.

Weinstein isn't the only U.S. citizen killed by a drone strike to be added to the list recently. The same operation also killed an American al-Qaeda militant named Ahmed Farouq, the White House said in a statement. Another strike in the same region in January killed Adam Gadahn, a prominent al-Qaeda figure who was also a U.S. citizen.

The U.S. government believes it can legally justify the killing of American citizens in drone strikes: In 2014, the Justice Department released a secret 2011 memo that explained the legal justification for killing American terror suspects living overseas without trial.

And Obama himself has acknowledged that he gave the order to kill Awlaki. "I would have detained and prosecuted Awlaki if we captured him before he carried out a plot, but we couldn’t," he said in a 2013 speech at the National Defense University. "And as president, I would have been derelict in my duty had I not authorized the strike that took him out."

But so far, Awlaki is the only U.S. citizen who was actually targeted by American drones. The other Americans killed by drone strikes listed above were believed to have been killed inadvertently. They were just in the wrong place at the wrong time, officials say, even when they were terror suspects.